1. Field of the Invention
This invention relates to a field mapping fixture for mapping the magnetic fields of a superconducting magnet. Such structures of this type, generally, are capable of measuring the magnetic fields of a relatively small imaging volume.
2. Description of the Related Art
In magnetic resonance (MR) imaging systems, the magnetic field inhomogeneities has to be less than a few parts per million (ppm) for producing good images. Therefore, a superconducting magnet is designed for a homogeneous field distribution of only a few (ppm) within the imaging volume. However, once built, due to the manufacturing tolerances, lead routings, and other manufacturing difficulties, the ppm inhomogeneities are usually in the range of hundreds. Active or passive shimming needs to be used to homogenize the magnetic field.
In recent years, due to its lower Cost, simplicity, and capabilities, passive shimming has been the preferred method of field inhomogeneity correction. However, due to the compact structure of an imaging system which images only human limbs, the space available for passive shimming is extremely limited. In order to keep the length of the MR system down and the inside diameter of the system large, the magnet coils and the gradient coil need to be physically very close. This is in fact the area where passive shims need to be placed. Therefore, a passive shimming system needs to be designed which will occupy the least amount of physical space.
While shimming the magnet, the magnetic field needs to be measured at a number of points over the surface of the imaging volume. The imaging volume of the superconducting magnet for imaging human limbs as set forth in U.S. patent application Ser. No. (08/000,344 filed Jan. 4, 1993 and pending (RD-22,167), entitled "SUPERCONDUCTING MAGNET FOR IMAGING HUMAN LIMBS" to Laskaris et al. and assigned to the same assignee as the present invention, is approximately a 10 centimeter diameter sphere. Therefore, a mapping fixture which has a low weight and can very accurately position the probe at all the measuring points of the small imaging volume is needed.
It is apparent from the above that there exists a need in the art for a mapping fixture which is light weight and compact through simplicity of parts and uniqueness of structure, and which at least equals the mapping characteristics of known mapping systems, but which at the same time is capable of mapping a relatively small imaging volume. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.